1. Field of the Invention
The present invention relates to a higher signal-to-noise ratio of a read-out signal in an optical disc apparatus.
2. Description of the Prior Art
As to optical discs, a Blu-ray Disc which is read and written by using a blue laser diode and a high NA (Numerical Aperture) objective lens has been commercialized. With this commercialization, it is considered that the optical discs have almost reached its limit in terms of the resolution of its optical system, and that, hereafter, a multi-layer structure in an optical disc is effective in order to further achieve a larger capacity in the optical disc. In a multi-layer optical disc, the amounts of light detected from respective layers need to be approximately the same. Thus, the reflectance from a specific layer has to be kept small. Meanwhile, along with the achievement in an increase of the capacity in the optical disc, a transfer rate has also been continuously increased because of the need to increase a dubbing speed for a video and the like. Accordingly, a sufficient signal-to-noise ratio of a read-out signal becomes less and less likely to be secured without modification. For this reason, a higher signal-to-noise ratio of a detection signal is required to achieve the multi-layer and the higher speed at the same time in the future.
Techniques related to the higher signal-to-noise ratio of a read-out signal of the optical disc are described, for example, in Japanese Patent Application Laid-Open Publications Nos. Hei 5 (1993)-342678, Hei 6 (1994)-223433 or the like. Both of the techniques are related to the higher signal-to-noise ratio of a read-out signal of a magneto-optical disk, and aim to achieve the following. Specifically, light from a laser diode is branched off before irradiation onto an optical disk. Then, light with which the optical disc is not irradiated is combined with and interferes with light reflected from the optical disk. Thus, the amplitude of a weak signal is increased by increasing the amount of the light with which the optical disc is not irradiated. In a case where a differential detection between transmitted light and reflected light is performed by a polarizing beam splitter heretofore used in a signal detection for a magneto-optical disk, the detection is performed essentially in the following manner. Specifically, original incident polarization components are allowed to interfere with polarization components orthogonal to an incident polarization direction. The orthogonal polarization components are generated by polarization rotation of the magneto-optical disk. In this way, the detection is performed by amplifying the orthogonal polarization components with the original incident polarization. Thus, the signal can be amplified by increasing the original incident polarization components. However, in order to prevent deletion and overwriting of data, the intensity of light made incident on the optical disk needs to be suppressed to a certain level or less. In contrast to this, in the conventional techniques described above, the intensity of light allowed to interfere with signal light for signal amplification can be increased regardless of the intensity of light on a surface of the disk in the following way. To be more precise, the light to be allowed to interfere with the signal light is previously separated. Then, the separated light is not collected on the disk, but allowed to interfere with the signal light. Thus, in principle, the more the intensity is increased within the allowable range of the intensity, the more the signal-to-noise ratio can be improved in relation to, for example, the noise of an amplifier which converts a photocurrent from a photo detector into a voltage, or the shot noise caused by the photo detector.
In Japanese Patent Application Laid-Open Publication No. Hei 5 (1993)-342678, two lights are allowed to interfere with each other to detect the interference intensity. In this event, securing of interference signal amplitude is intended by varying an optical path length of disk-nonreflective light to interfere with the other light. In Japanese Patent Application Laid-Open Publication No. Hei 6 (1994)-223433, the differential detection is also performed, in addition to the interference intensity detection. Thus, a higher signal-to-noise ratio is realized by canceling intensity components of the respective lights, which do not contribute to a signal, and by canceling noise components of the lights. For the differential detection in this case, a non-polarizing beam splitter is used.